Kimberlite and Lamproite

The multifaceted volcanic magmas that solidify into kimberlite and lamproite are not the source of diamonds, only the elevators that bring them with other minerals and mantle rocks to Earth's surface. Though rising from much greater depths than other magmas, these pipes and volcanic cones are relatively small and rare, but they erupt in extraordinary supersonic explosions.

Kimberlite and lamproite are related mixtures of rock material. Their important constituents include fragments of rock from Earth's mantle, large crystals, and the crystallized magma that glues the combination together. The magmas are very prosperous in magnesium and volatile compounds such as water and carbon dioxide. As the volatiles dissolved in the magma change to gas near Earth's surface, explosive eruptions make the characteristic carrot- or bowl-shaped pipes.

Kimberlite magma rises during Earth's crust in networks of cracks or dikes. The pipes only form close to Earth's surface. This cross-section of a kimberlite pipe shows the carrot-shaped profile formed by explosive eruption. The root zone starts in fissures, where gases are free from the rising magma and drive the eruption; they blow out the fragment-laden kimberlite to form the volcano's tuff ring and fill the pipe. Depth measurements demonstrate the level of erosion for various kimberlite pipes in South Africa. Modified from Hawthorne (1975).